1. Field of the Invention
The present invention is related to integrated circuit manufacturing, and more particularly to the use of diffraction tomography for latent image metrology.
2. Background Information
Integrated circuits are manufactured through a sequence of controlled steps used to form layers of conducting, insulating and transistor-forming materials. Layers within the integrated circuit are formed by covering the surface of a wafer with photoresist and exposing selected portions of the photoresist to light at a particular wavelength.
The exposure process begins when an exposing light source, traveling through a photomask containing the desired patterns, falls incident on the wafer. The photoresist is exposed to the light source traveling through the photomask, and depending on the photomask""s geometry, the photoresist consequently undergoes gradual refractive index changes between the exposed region and the unexposed region. The changes in the photoresist due to exposure may not be visible at this point. That is why the image is termed a latent image. The photoresist is then developed and the unexposed areas are washed away.
Latent image formation is a critical time in the manufacturing process. If the photoresist has been underexposed, the features formed in the developed photoresist may be too small to perform correctly. To date, the methods used to monitor latent image formation have been limited to either microscope inspection or diffraction measurement.
Microscope inspection cannot be done real time. Microscope inspection typically requires a complicated setup. Furthermore, the wafer usually must be taken from the fab line to the microscope to be inspected.
Diffraction measurement techniques used to date have been limited to heuristic approaches which correlate the intensity of diffractions to final critical dimension (CD) measurements. For instance, one would measure intensities of the first diffraction order and, after development of the photoresist, measure the resulting critical dimension. Such an approach is very inflexible. That is, any change to manufacturing parameters such as exposure intensity or duration, or in the chemistry of the photoresist, forces the manufacturer to go through the entire correlation process all over again.
What is needed is a system and method for monitoring latent image formation which can be done in close to real time, yet which is flexible enough to accommodate changes in the manufacturing process.
The present invention is a system and method of reconstructing an image of a structure having periodic variations in index of refraction. Electromagnetic waves are projected onto the structure and the resulting diffracted electromagnetic waves are measured, wherein the step of measuring includes the step of determining a plurality of intensities DE. Refractive terms can then be calculated as a function of the intensities DE.
According to one aspect of the present invention latent image formation in photoresist can be measured by forming a periodic structure such as a grating in the photoresist. Light is projected onto the structure and the resulting diffracted light is measured, wherein the step of measuring includes the step of determining a plurality of intensities DE. Refractive terms can then be calculated as a function of the intensities DE.
According to another aspect of the present invention, a stepper can be constructed which can monitor latent image formation in photoresist. The stepper includes a fixture for receiving a wafer and a photomask, a first light source for illuminating the wafer through the photomask and at a first incident angle, a second light source positioned to illuminate the wafer at a second incident angle, wherein the second angle is different from the first incident angle and a detector positioned to measure light diffracted from the wafer.
According to yet another aspect of the present invention, a track can be constructed which can monitor latent image formation in photoresist. The track includes a bake chamber, a development chamber, a light source positioned to illuminate the wafer at an incident angle and a detector positioned to measure light diffracted from the wafer.